The present invention relates to rotary drilling apparatus and more particularly to an improved kelly and a drive bushing for rotating the kelly.
As is well known in oil well drilling, a swivel supported kelly is rotatably driven by a kelly bushing which also permits translational movement of the kelly into a well hole. The driving torque for the kelly is transmitted by a rotary table driven master bushing to the kelly bushing through either a pin or square type drive forming part of the kelly bushing.
Due to the tremendous magnitude of weight and torque imposed on the kelly during the drilling operation, a kelly needs to be strong in tension and in torsion as well as to be equally strong in bending in all directions. As is well known, a slightly bent kelly will produce enough resistance to the longitudinal movement of the kelly through the drive bushing to prohibit its passage therethrough. Furthermore, a slight bend in the kelly causes misalignment of the drill pipe in the bore hole resulting in an uncontrolled wobble that produces deleterious forces that can cause severe damage to the various components of the drilling apparatus.
For most bore hole dimensions, the maximum strength kelly is yielded by a kelly having a circular cross-sectional area such as the one shown in U.S. Pat. No. 2,202,446. Although, the circular kelly is recognized as one of high strength, it suffers from the problem of the absence of practical means for rotating same. Thus, over the years, several other shaped kellys have been developed; such as: a circular kelly provided with longitudinally extending flutes or grooves as illustrated in U.S. Pat. Nos. 2,620,163; 2,338,093 and 2,859,939; a circular kelly provided with longitudinal extending external ribs as exemplified in U.S. Pat. Nos. 1,067,330 and 2,829,866; and polygonal cross-sectional kellys having a square shape as illustrated in U.S. Pat. No. 3,527,064 and those of the hexagonal shape as illustrated in U.S. Pat. No. 2,338,093.
Among the most popular of these kellys are those having the square or hexagonal shape. Both of these shaped kellys are normally driven by a drive bushing having four rollers. In the case of the square kelly, each of the four rollers are essentially cylindrical and equally spaced about the axis of the bushing and so positioned to driveably engage respective sides of the kelly. In the case of the hex kelly, two of the cylindrical rollers are replaced with V-shaped rollers such that the two remaining cylindrical rollers driveably engage two diametrically opposite sides of the kelly whereas the two diametrically opposed V-shaped rollers driveably engage the corresponding adjacent sides of the kelly at the corners formed thereby which theoretically results in roller engagement of all six sides of the hex kelly. In such drive arrangements it has been found that instead of having a line contact between the rollers and sides of the kelly there is actually a point contact. This is largely due to wear of the surfaces of the components after limited use and due to the fact that clearance space is required between the rollers and the kelly to permit longitudinal movement of the latter through the bushing during the drilling operation. Such point contact normally occurs at or very near the corners of the kelly during the driving operation, and thus after continued used, the corners of the kelly wear to a degree which decreases the frictional drive engagement by the rollers to a point where slippage occurs resulting in damage to the rollers and replacement of the kelly.
Thus, it is desirable to have a kelly for a given bore hole having a cross-sectional area that approaches that of a circular kelly and which is greater than the cross-sectional area of a standard square or hexagonal kelly so as to maximize the strength of the kelly. Further, it is desirable that the kelly has flat drive surfaces similar to the square and hex kelly to facilitate the driving of same which is one of the shortcomings of the circular kelly. Furthermore, it is desirable that the kelly has a sufficient cross-sectional area that the corners thereof are of sufficient strength so as to prevent excessive wear thereto due to the point drive contact of the rollers to thereby prolong the useful life of the kelly. Still further it is desirable to have a kelly drive bushing that driveably cooperates with the kelly such that the bushing rollers are in drive engagement with the flat drive surfaces of the kelly simultaneously.